Phosphines are used as ligands in a large number of known transition metal complexes, and phosphine ligands are included in many transition metal complexes used as catalysts. One of the reasons is that phosphines are known to be one of the best ligands for stabilizing transition metals. Phosphine ligands are often included in transition metal complexes used to catalyze hydroformylation reactions where hydrogen, an alkene, and carbon monoxide are converted to the corresponding aldehyde.
Phosphines are also included as ligands in various transition metal complexes used to catalyze hydrogenation reactions. In many of these reactions, inexpensive phosphines such as triphenylphosphine perform suitably. However, phosphines have also found a niche in more specialized areas such asymmetric hydrogenation and other catalytic transformations. The use of a chiral phosphine allows enantioselectivity in the catalytic reaction, and often high enantiomeric excesses may be achieved when a chiral phosphine is used as a ligand. The use of an enantioselective catalyst allows a desired enantiomer to be produced reducing undesired products while simultaneously reducing separation costs associated with the separation of enantiomers. Enantioselective hydrogenation catalysts may be as fast and selective as some of the best known enzymes, and such catalysts can result in greater than 99.9% production of one enantiomer.
Asymmetric hydrogenation is used to make commercially important products including biologically active compounds such as pesticides and pharmaceuticals. Asymmetric hydrogenation is being used more frequently in the pharmaceutical industry where expensive intermediate compounds are too valuable to waste. One of the first reactions employing a phosphine-containing catalyst in the pharmaceutical industry was the selective production of L-DOPA rather than R-DOPA.
As noted above, chiral phosphine ligands are central to many developments in transition metal-catalyzed enantioselective transformations. R. Noyori, Asymmetric Catalysis; John Wiley: New York, 1994. Recent demonstrations of high enantioselectivity for a wide range of hydrogenation reactions with Rh complexes of the DuPHOS, PennPHOS, RoPHOS, BASPHOS, CnrPHOS, and related ligands highlight the unusual efficacy of rigid phosphacycles. M. J. Burk, J. Am. Chem. Soc. 1991, 113, 8518-8519; M. J. Burk, Chemtracts-Organic Chemistry 1998, 11, 787-802; M. J. Burk, A. Pizzano, J. A. Martin, L. M. Liable-Sands, A. L. Rheingold, Organometallics 2000, 19, 250-260; M. J. Burk, F. Bienewald, S. Challenger, A. Derrick, J. A. Ramsden, J. Org. Chem. 1999, 64, 3290-3298; Z. Zhang, G. Zhu, Q. Jiang, D. Xiao, X. Zhang, J. Org. Chem. 1999, 64, 1774-1775; Q. Jiang, Y. Jiang, D. Xiao, P. Cao, X. Zhang, Angew. Chem. 1998, 110, 1100-1103; Angew. Chem., Int. Ed. Engl 1998, 37, 1100-1103; G. Zhu, P. Cao, Q. Jiang, X. Zhang, J. Am. Chem. Soc. 1997, 119, 1799-1800; Z. Chen, Q. Jiang, G. Zhu, D. Xiao, P. Cao, C. Guo, X. Zhang, J. Org. Chem. 1997, 62, 4521-4523; J. Holz, M. Quirmbach, U. Schmidt, D. Heller, R. Stürmer, A. Börner, J. Org. Chem. 1998, 63, 8031-8034; W. Li, Z. Zhang, D. Xiao, X. Zhang, J. Org. Chem. 2000, 65, 3489-3496; W. Li, Z. Zhang, D. Xiao, X. Zhang, Tetrahedron Lett. 1999, 40, 6701-6704; Y. -Y. Yan, T. V. RajanBabu, J. Org. Chem. 2000, 65, 900-906; J. Holz, D. Heller, R. Stürmer, A. Börner, Tetrahedron Lett. 1999, 40, 7059-7062; A. Marinetti, S. Jus, J. -P. Genêt, Tetrahedron Lett. 1999, 40, 8365-8368; A. Marinetti, S. Jus, J. -P. Genêt, L. Ricard, Tetrahedron 2000, 56, 95-100; A. Marinetti, S. Jus, J. -P. Genêt, Tetrahedron Lett. 1999, 40, 8365-8368; A. Marinetti, S. Jus, J. -P. Genêt, L. Ricard, Tetrahedron 2000, 56, 95-100.
Although significant efforts have been made to produce transition metal complexes for effecting enantioselective catalytic transformations, one persisting problem associated with chiral phosphine ligands is that they are difficult and expensive to produce, often requiring multi-step syntheses. Both the electron density of the phosphorus atom in phosphines and the size of the phosphine ligand as expressed by cone angles are known to impact the reactivity of metal complexes prepared from them. Therefore, the ability to modify chiral phosphines and determine structure property relationships are important factors in understanding and optimizing catalytic activity. However, the difficulty associated with synthesizing chiral phosphines has prevented the synthesis of libraries of such compounds for use in analyzing structure property relationships.
One specific group of phosphines, 3,4-diazaphospholanes, are five-membered rings containing two nitrogen atoms, two carbon atoms, and a phosphorus atom as ring members. In 3,4-diazaphospholanes, each of the two carbon atom ring members is bonded to one of the ring nitrogen atoms and the ring phosphorus atom. Very few 3,4-diazaphospholanes have thus far been reported.
Märkl et al. have prepared diazaphospholanes by reacting hydrazines with phosphorus compounds having the formula RP(CH2OH)2. This synthetic methodology is limited and does not provide any simple route to compounds having groups other than H bonded to the diazaphospholane ring carbon atoms. G. Märkl, G. Y. Jin, Tetrahedron Lett. 1980, 21, 3467-3470; and G. Märkl, G. Y. Jin, Tetrahedron Lett. 1981, 22, 229-232. Arbuzov et al. have utilized the same type of methodology to prepare other diazaphosphacycles from RP(CH2OH)2. B. A. Arbuzov, O. A. Erastov, G. N. Nikonov, R. P. Arshinova, I. P. Romanova, R A. Kadyrov, Izvestia Akad, Nauk SSSR, Seriya Khimicheskaya, 1993, 8, 1846-1850.
A need remains for chiral phosphines and methods for making them. A need also remains for transition metal complexes that include chiral phosphines and for transition metal complexes for catalyzing important reactions. A need further remains for libraries of chiral phosphines and transition metal complexes.